Abstract

We report on the discovery and novel physics of a new superconductivity dome in LaFeAsO1−xFx with high-doping rate (0.25 ≤x≤0.75) synthesized by using the high-pressure technique. The maximal critical temperature Tc = 30 K peaked at xopt = 0.5 ∼0.55, which is even higher than that at x≤ 0.2. By nuclear magnetic resonance (NMR), we find that the new superconducting dome is far away from a magnetically ordered phase without low-energy magnetic fluctuations. Instead, NMR and transmission electron microscopy measurements indicate that a C4 rotation symmetry-breaking structural transition takes place for x> 0.5 above Tc. The electrical resistivity shows a temperature-linear behavior around the doping level where the crystal transition temperature extrapolate to zero and Tc is the maximal, suggesting the importance of quantum fluctuations associated with the structural transition. Our results point to a new paradigm of high temperature superconductivity.